The present invention relates to a CRT panel glass (hereinafter also referred to simply as a panel glass) having the contrast and uniformity in brightness improved and having occurrence of a double image suppressed, and a CRT provided with such a panel glass and being excellent in the above characteristics.
A panel glass for CRT is required to present a uniform brightness of an image displayed in the picture plane.
As a method to display an image brightness uniformly, a method wherein the transmittance of the panel glass is made constant in the plane, or a method wherein, while a transmittance distribution is permitted to be present in the panel glass, it is corrected by an intensity distribution of an electron beam, and a distribution is imparted to the emission intensity of the phosphor, may be mentioned.
However, the latter method has a technical limit such that when a panel glass has a large transmittance distribution, such as a transmittance distribution of at least 10%, it cannot cope therewith.
On the other hand, as a conventional method wherein the transmittance of a panel glass is made constant, the glass base material is made to be transparent to eliminate the difference in the transmittance due to the thickness of the glass. However, this method cannot cope with flattening of recent panel glasses where the difference in thickness between the center portion and the peripheral portion is large, and it is difficult to make the transmittance of such panel glasses to be uniform. Further, it has been attempted to solve the above problems by increasing the transmittance of the glass base material as a panel glass provided with an antireflection film and by reducing the transmittance of the antireflection film, but there has been a problem that the internal reflection tends to be high, and a double image is likely to form. Consequently, in a case of a flattened panel glass, there have been problems such that the contrast is poor, and a double image forms, in addition to a problem that uniformity cannot be attained in the brightness of images.
Further, as another related technology, JP-A-61-185852 discloses a method wherein the total glass thickness within the display area is made constant by attaching a front panel made of glass and provided with a thickness variation on the front surface of the panel glass by a resin. However, this method has had a problem that the weight of CRT increases, or the cost increases due to the bonding of the front panel.
Further, JP-A-6-308614 discloses a projection type display wherein, in order to improve the contrast, the projection screen is made to be a colored screen having an outer light absorption characteristic on its surface, and the color density is continuously changed from the center towards the periphery, so that the brightness distribution of the screen will be uniform, as seen by an observer. This publication also discloses that a similar effect can be obtained also by providing a distribution to the thickness of the colored screen. These methods are considered to be effective as means to make the brightness over the entire surface of the display uniform by intentionally providing a transmittance distribution to the display. However, in a case where such methods are applied to a panel glass for CRT, it will be required to have a resin film or a front panel corresponding to the colored screen, thus leading to a problem of the weight increase or the increase of the costs, as in the case described above.
Further, JP-A-10-177850 discloses a method for making the transmittance of the center portion and the peripheral portion uniform by bonding a resin film on the front surface of a panel glass for CRT and by adopting any one of {circle around (1)} coloring the resin film, {circle around (2)} applying a colored coating on the resin film surface and {circle around (3)} coloring the adhesive used for bonding the resin film to the panel glass. However, this method also has had a problem of the weight increase or the increase of the costs.
The object of the present invention is to provide a panel glass whereby a uniform brightness image can be observed with a good contrast, even with a flattened panel glass whereby the difference in brightness between the center portion and the peripheral portion becomes distinct especially when a colored glass is employed.
The present invention further has an object to provide a method for producing a panel glass, whereby the above-mentioned panel glass can be obtained in a simple method at a low cost.
The present invention further has an object to provide a panel glass having also an electromagnetic wave shielding performance and a method for its production.
The present invention further has an object to provide a panel glass having also a low reflection performance and a method for its production.
Further, the present invention has an object to provide a panel glass having occurrence of a double image suppressed and a method for its production.
Another object of the present invention is to provide a CRT provided with a panel glass having the above-mentioned excellent characteristics.
According to the present invention, panel glasses of the following 1) to 20), a method for producing a panel glass of 21), and a CRT of 22) are provided, whereby the above-mentioned objects of the present invention are accomplished.
1) A panel glass having a surface treating film formed on an outer surface of a substrate glass, characterized in that the surface treating film is formed on the outer surface so that the value A defined by the following mathematical expression (1) is less than 1 within an effective picture plane:                               Value  A                =                  "LeftBracketingBar"                                    1              -                                                Tgf                  ⁡                                      (                    min                    )                                                                    Tgf                  ⁡                                      (                    max                    )                                                                                      1              -                                                Tg                  ⁡                                      (                    min                    )                                                                    Tg                  ⁡                                      (                    max                    )                                                                                "RightBracketingBar"                                    Mathematical  expression  (1)            
In the mathematical expression (1):
Tg(min) and Tg(max) represent the minimum transmittance (%) and the maximum transmittance (%) of the substrate glass itself, respectively; and
Tgf(max) and Tgf(min) represent the maximum transmittance (%) and the minimum transmittance (%) of an integral body comprising the substrate glass and the surface treating film.
2) The panel glass according to 1), characterized in that the value A is at most 0.85.
3) The panel glass according to 1) or 2), characterized in that the value A is at most 0.7.
4) The panel glass according to any one of 1) to 3), characterized in that the difference between the minimum transmittance Tf(min) of the surface treating film and the maximum transmittance Tf(max) of the surface treating film is within a range of from 2 to 20%.
5) The panel glass according to any one of 1) to 4), characterized in that the difference between Tf(min) and Tf(max) is within a range of from 3 to 10%.
6) The panel glass according to any one of 1) to 5), characterized in that Tgf(min)/Tgf(max) is at least 0.8.
7) The panel glass according to any one of 1) to 6), characterized in that Tgf(min)/Tgf(max) is at least 0.9.
8) The panel glass according to any one of 1) to 7), characterized in that Tgf(min)/Tgf(max) is at least 0.95.
9) The panel glass according to any one of 1) to 8), characterized in that the surface treating film is an antireflection film against outer light.
10) The panel glass according to any one of 1) to 9), characterized in that the surface treating film is a light absorptive film.
11) The panel glass according to any one of 1) to 10), characterized in that at least one layer among layers constituting the surface treating film is a conductive film.
12) The panel glass according to any one of 1) to 11), characterized in that the change in the thickness of the substrate glass is distributed mainly in the long axial direction of the panel glass.
13) The panel glass according to any one of 1) to 12), characterized in that the change in the thickness of the substrate glass is distributed mainly in a short axial direction of the panel glass.
14) The panel glass according to any one of 1) to 13), characterized in that Tg(max) is within a range of from 30 to 70%.
15) The panel glass according to any one of 1) to 14), characterized in that the reflectance as observed from inside of the substrate glass, is at most 15%.
16) The panel glass according to any one of 1) to 15), characterized in that the surface treating film is a film having a construction such that a light absorptive layer containing titanium nitride as the main component and a layer containing silica as the main component, are formed in this order on the substrate glass.
17) The panel glass according to 16), characterized in that the light absorptive layer containing titanium nitride as the main component, has a thickness distribution.
18) The panel glass according to 17), characterized in that the layer containing silica as the main component has a thickness distribution diverse to the light absorptive layer containing titanium nitride as the main component.
19) The panel glass according to any one of 1) to 18), wherein Tgf(max) is within a range of from 30 to 70%.
20) The panel glass according to any one of 1) to 19), characterized in that the transmittance Tf at an optional position of the surface treating film is within a range of from 40 to 90%.
21) The panel glass according to any one of 1) to 20), characterized in that Tf is within a range of from 60 to 90%.
22) The panel glass according to any one of 1) to 21), characterized in that the outer diameter of the panel glass (the average radius of curvature of the outer surface of the panel glass) is at least 5 times the value R calculated by the following mathematical expression (2):
Value R (unit: mm)=Screen diagonal length (inch)xc3x9742.5+45.0 xe2x80x83xe2x80x83Mathematical expression (2) 
(provided that the screen diagonal length is the effective picture plane size (inch) of the display.)
23) A method for producing a panel glass characterized in that a surface treating film is formed on an outer surface of a substrate glass, characterized in that the surface treating film is formed on the outer surface so that the value A defined by the mathematical expression (1) is less than 1 within an effective picture plane.
24) A CRT having the panel glass as defined in any one of 1) to 22).